There is now a consensus that the mean temperature of the Earth will increase by an average of 2.0œ to 5.8œ Celsius in the twenty-first century. Some environmental models, taking into account likely changes in vegetation cover, predict an even higher rise of 8œ Celsius during the century. The resulting elevated temperatures (even at the lower end of the estimates) will have significant effects on Earth's biosphere, including human life. Many factors are associated with this predicted temperature rise, but agriculture is among the major contributors.

Agricultural activity is a significant source of greenhouse gases (GHGs). GHG levels are affected by land clearing, high energy inputs, soil degradation, and intensive animal husbandry. Based on current estimates, agriculture contributes to 25 percent of the world's carbon dioxide (CO2) emissions, 60 percent of methane gas emissions, and 80 percent of nitrous oxide emissions. Agriculture's high energy input results primarily from manufacturing chemical fertilizers, herbicides, and pesticides; operating farm machinery; irrigating farmland using pumps and other machines; and transporting products over long distances. Collectively, these activities account for more than 90 percent of the total energy expenditure in agriculture.

The burning of fossil fuels releases CO2 into the atmosphere. The CO2 concentration in the atmosphere has increased from 277 parts per million to 382 parts per million since the beginning of the Industrial Revolution in the mid-eighteenth century. Industrialized agriculture is believed to have contributed to 25 percent of that increase.

Overuse of fertilizers, in addition to energy inputs in fertilizer manufacturing, contributes significantly to climate change. More than half of all synthetic fertilizers applied to the soil either end up in local waterways or emit to the atmosphere. A portion of the excess nitrogen fertilizers in the soil is converted into nitrous oxide, which is 296 times more potent than CO2 in trapping heat and which has a long atmospheric lifetime of 114 years. Each year, nitrous oxide emissions alone account for the equivalent of 1.9 billion metric tons of CO2 emissions.

The second greatest GHG emission by agriculture is methane, released in small amounts by rice paddies and in much larger amounts by livestock. As the demand for meat increases, more livestock are raised and are fed higher-protein diets. Both the number of livestock and their protein-rich diets increase the amount of methane they emit. Methane gas is fourteen times more potent than CO2 in trapping heat. Its concentration has almost tripled since the Industrial Revolution, from 600 parts per billion to 1,728 parts per billion.

Potential climate changes associated with elevated GHGs and an altered surface energy budget include an increased incidence of heat waves, severe storms, and floods, as well as elevated sea levels. Some 30 percent of the agricultural lands worldwide could be affected by these changes. Global warming alone is projected to have considerable effects on agriculture. A warming of 2œ Celsius or more could reduce global food supplies and aggravate world hunger. The impact on crop yields will vary considerably across different agricultural regions. Warm regions, such as tropics and subtropics, will be threatened by climate change, while cooler regions, mainly in temperate or higher latitudes, may benefit from warming.

Global climate change may have significant effects upon livestock systems as well. First, the productivity and quality of rangelands may be adversely affected. This in turn will affect the quality and productivity of livestock. Second, higher grain prices resulting from the disruption of crop production will lead to higher costs for livestock products. Third, increased severity and frequency of storms may intensify soil erosion and decrease the productivity of rangelands. Fourth, global warming could result in changes in the distribution and severity of livestock diseases and parasites, which may threaten the health of animals, especially those in intensively managed livestock systems.

Unlike any other industrial GHG emitters, agriculture has the potential to change from being one of the largest GHG sources to being a net carbon sink, reversing its role in climate change. Several practical measures can be taken to mitigate the climate change caused by intensive agriculture. These include the reduced and more efficient use of chemical fertilizers, protection of soil, improvement of paddy rice production, and reduction of demand for meat.

Precision farming can reduce the need for chemical fertilizers. In precision farming, fertilizers and other agrochemicals are applied based on crops' needs, in precise amounts and on a carefully managed schedule. The reduced application of these chemicals not only cuts GHG emissions but also alleviates other environmental problems such as water pollution and eutrophication of waterways.

As a result of intensive farming, agricultural soils have some of the lowest carbon contents of all land types. If these soils can be modified to absorb more of Earth's carbon, the result will be a net reduction in atmospheric carbon. Low soil carbon content can be reversed through a number of measures, including planting cover crops, fallowing, and engaging in conservation tillage. These practices will increase the amount of organic matter (and thus the carbon content) in the soil. They will also reduce soil erosion and surface runoff, thereby reducing the need for chemical fertilizers. Collectively, these measures can turn agricultural soils into carbon sinks, changing the nature of their impact on climate change.

To reduce methane emissions from rice production, better cultivation techniques will need to be adapted. For example, rather than continuously flooding rice paddies, farmers could supply water to the paddies only when it is needed during the growing season and keep the paddies dry during the non-growing season. Such measures could reduce methane emission from rice fields significantly.

Livestock raising is the second largest source of GHGs in agriculture. The most efficient way to cut methane emission due to livestock is simply to reduce the number of farm animals. As an ever-increasing demand for meat and dairy products drives increasing animal husbandry, one effective approach to cut methane emission is to reduce the demand for meat, especially in developed countries where consumers have tremendous buying power. Reduced meat and dairy consumption would go a long way toward curbing methane emissions.

Agriculture and climate change are interlocked processes, in that each exerts effects on the other in a complex fashion. Climate changes, especially shifts in precipitation and temperature, are widely believed to have significant effects on agriculture, because these two factors determine the carrying capacity of any ecosystem. At the same time, modern agriculture is a major contributing factor to global warming, as altered land cover and the emission of CO2, methane gas, and nitrous oxide from intensive farming increase the GHG content of the atmosphere. However, it remains possible to transform industrialized agriculture, using techniques that could render it more sustainable and mitigate its effects upon global and local climates.

Bibliography:

1)         Bazzaz, Fakhri, and Wim Sombroek, eds. Global Climate Change and Agricultural Production: Direct and Indirect Effects of Changing Hydrological, Pedological, and Plant Physiological Processes. New York: Wiley, 1996.

2)         Marland, Gregg, et al. "The Climatic Impacts of Land Surface Change and Carbon Management, and the Implications for Climate-Change Mitigation Policy." Climate Policy 3 (2003): 149-157.

3)         Palo, Matti, and Heidi Vanhanen, eds. World Forests from Deforestation to Transition? Boston: Kluwer Academic, 2000.

4)         Paustian, Keith, et al. Agriculture's Role in Greenhouse Gas Mitigation. Arlington, Va.: Pew Center on Global Climate Change, 2006.

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